The invention relates to a device for protecting individually from electrocution and more specifically to a differential transformer which delivers a trigerring current when a current difference occurs between two phases of a multiphase distribution mains.
The invention has for its object a reduction in cost of such devices. One of the expensive parts is the magnetic torus forming the armature of the differential transformer, and the object of the invention is to provide a differential transformer of reduced volume, and consequently of lower cost.
The working method of such a device is well known. FIG. 1 of the appended drawings is a schematic representation of a differential transformer used in a monophase circuit. Phase conductors 1, 2 of input main E are coupled to a useful load R, and include coils 3 and 4, having the same number of turns, but in opposite directions, so that the fields of these coils in torus 5 are equal and opposite. Consequently, there is no current induced in the secondary coil 6. If conductor 1 happens to have a failure to earth, eg. somebody is in touch at 8 with conductor 1, which provides a connection to earth in 9 with an equivalent resistor .rho., currents passing in coils 3 and 4 are no longer equal and a voltage arises in the secondary coil 6. Then a current passes through the secondary circuit, and this current is used to cause the operation of a relay, triggering a safety or protective device, such as a circuit breaker. For the safety of human beings, for the protection of individuals from electrocution it has been settled that the extra current to earth should not rise above 30 mA. This value represents the difference between the currents passing through conductors 1 and 2 and should give rise to a current in the secondary coil sufficient for allowing the direct triggering of a sensitive actuator, such as the one disclosed in French Patent No. 75 34654 (Publication No. 2 331 877) or eventually, in addition with storage or amplifying means, such as those described in French Patents Nos. 1,323,673, 1,347,117 and 1,411,747.
For designing such a device, the first point to be considered is the threshold primary differential current, i.e. the 30 mA cited in the above example, or 450 mA, value frequently used for the safety of installations, or 6 mA, limit value used for the safety relating to electrocution in the United States. It can be understood that the operation of the magnetic torus is finally determined by the number of Ampere-turns per length of magnetic circuit. That is to say for a given torus, determined by the product of this limit differential current per the number of turns of each primary coil. This product should be so much greater than the torus is more magnetized, since it has to deliver a power as high as possible.
Other independent factors are the number of phases of the mains (two in the example of FIG. 1, three for a triphase distribution, and four for a triphase distribution with neutral), the phase nominal current defining the maximum temperature rise of the coils (3 and 4 on FIG. 1) and then of torus 5, the maximum phase short-circuit current, defining the strength of the conductors, regarding fusibility.
The resistance of coils 3 and 4 gives rise to a Joule effect. The resulting heating should not be too high, since the heating, for a given nominal phase current, is directly proportional to the number of turns of each primary phase.
A first discrepancy now appears: for the proper operation of the magnetic torus, the number of primary turns should be as high as possible, but unfortunately, that causes too many Joule effect losses.
Further, this number of turns, with as large a section area as possible to obtain a low resistance is desired, is to be passed in the central aperture of torus. There appears a second discrepancy: the torus should have a larger diameter, which reduces the magnetization, since the magnetic circuit is longer.